A storm surge is the abnormal rise of sea level that a cyclone pushes onto the coast, and along India’s shores, especially around the Bay of Bengal, it has historically been the deadliest part of a tropical cyclone, far more lethal than the wind. The surge is a dome of sea water, raised by the cyclone’s winds and low pressure, that sweeps inland over low coastal ground in minutes and can reach several kilometres beyond the beach.
Understanding the storm surge explains why a cyclone with moderate winds can still drown thousands, why the head of the Bay of Bengal is so dangerous, and why evacuation, not strong buildings alone, saves lives. It also explains why Odisha, which lost about ten thousand people in the 1999 super cyclone, now moves around a million people or more ahead of a major storm with very few deaths. This guide covers how a surge forms, why India’s coasts are exposed, the major historical events, and how forecasts and shelters work.
Quick Facts
| Phenomenon | Storm surge, the rise of coastal sea level above the normal astronomical tide caused by a cyclone |
|---|---|
| Main causes | Wind stress pushing water ashore, the pressure drop at the centre of the storm, and waves |
| Typical heights | From under one metre for weak storms to well over five metres for severe cyclones, with extreme historic cases higher still |
| Storm tide | The total water level when the surge combines with the normal tide |
| Most exposed Indian coasts | West Bengal, Odisha and Andhra Pradesh on the east, and Gujarat on the west |
| Amplifying features | Shallow, wide continental shelf, funnel-shaped bays and delta coasts, high tide at landfall and low-lying land |
| Strongest side of a cyclone | The right-hand side of the track (looking along the direction of motion) in the Northern Hemisphere |
| Landmark events | 1970 Bhola cyclone (Bangladesh), 1977 Andhra Pradesh cyclone, 1999 Odisha super cyclone, Amphan in 2020 |
| Forecasting bodies | India Meteorological Department (IMD) and the Indian National Centre for Ocean Information Services (INCOIS) |
Key Takeaways
- A storm surge is the extra height of sea water raised by a cyclone’s winds and low pressure, and it is historically the largest cause of cyclone deaths on the Bay of Bengal coast.
- The shallow, wide shelf and the funnel shape at the head of the Bay of Bengal let the surge pile up, which is why West Bengal, Odisha and Bangladesh have seen some of the worst surge disasters in recorded history.
- Surge height depends on the storm’s intensity, speed, size, angle of approach and the shape of the sea floor, and landfall at high tide can make the flooding much worse.
- The 1970 Bhola cyclone and the 1999 Odisha super cyclone are the two best-known examples, with the former among the deadliest cyclones ever recorded.
- Modern forecasts of storm surge, early evacuation and multipurpose cyclone shelters have sharply reduced deaths in Odisha since 1999 despite equally powerful storms.
- Sea-level rise, coastal subsidence and the loss of mangroves can increase future surge flooding, even if the number of cyclones does not change.
What is a storm surge?
A storm surge is the difference between the actual sea level during a cyclone and the level that the tide would normally have reached, produced by the storm’s winds and low atmospheric pressure. If the normal high tide is 2 metres and the sea actually reaches 5 metres, the surge is 3 metres.
Forecasters distinguish surge from storm tide. The storm tide is the total water level, which is the surge plus the normal astronomical tide at that moment. Flooding on land depends on the storm tide and on the height of the ground, so the same surge is far more dangerous at high tide than at low tide. Along the Sundarbans, where the tidal range is several metres, the timing of landfall can be the difference between a damaging flood and a catastrophic one.
A surge is also different from a tsunami or a normal wave. It is a slow, steady rise of the entire sea surface, driven by the atmosphere, and the waves ride on top of it. That is why the water keeps rising for hours as the cyclone approaches rather than arriving as a single crest.
How does a cyclone create a storm surge?
A cyclone creates a storm surge mainly by wind stress, as the strong onshore winds drag the surface water towards the coast and make it pile up against the land, helped by a smaller contribution from the low pressure at the storm’s centre. Waves breaking near the shore add a further rise, called wave set-up.
- Wind stress: sustained winds blowing toward the coast push water in the direction of the wind, and this is usually the largest part of the surge.
- Low pressure: the sea surface rises about a centimetre for each hectopascal drop in pressure, so a deep cyclone with a central pressure far below normal can lift the sea by a metre or more.
- Wave set-up: large breaking waves transfer energy to the water near the shore and raise its mean level.
- Coastal geometry: shallow water slows the flow back to sea, so the pushed-up water builds higher where the bottom rises gently.
- Earth’s rotation: the Coriolis effect turns currents in the cyclone to the right in the Northern Hemisphere, adding to the rise along certain coasts.
The highest surge generally occurs to the right of the point where the eye crosses the coast, because winds there blow most strongly onshore, being the sum of the storm’s rotation and its forward movement. This is why the exact landfall location, even a difference of tens of kilometres, matters so much for who is flooded.
Why is the Bay of Bengal so prone to storm surges?
The Bay of Bengal is so prone to storm surge because its northern end is a shallow, funnel-shaped basin fringed by a low, flat delta, which allows water driven by cyclone winds to build up and flood vast areas. Warm sea surface temperatures also let cyclones intensify there, and the large rivers of the Ganga, Brahmaputra and Meghna end in this delta.
The continental shelf is wide at the head of the Bay, which means the sea floor slopes very gently for a long distance. In shallow water, the same wind produces a bigger surge than in deep water, because there is less depth for the water to escape. The funnel shape squeezes the rising water into a narrowing space, much as a tide is amplified in a narrowing estuary. Delta land, often only a metre or two above sea level, offers little resistance and floods across a wide area.
Population density is the final factor. The coastal districts of Bengal, Odisha and Bangladesh are among the most densely populated low-lying places on Earth, so any given surge affects a very large number of people, houses and farms. The Arabian Sea coast has seen fewer surge disasters, but Gujarat, with its shallow coast and creeks, remains exposed, as past events like the 1998 Kandla cyclone showed.
What were the worst storm surge disasters in the Bay of Bengal?
The worst storm surge disasters in the Bay of Bengal include the 1970 Bhola cyclone, the deadliest tropical cyclone on record, the 1977 Andhra Pradesh cyclone and the 1999 Odisha super cyclone, each of which killed thousands or more mainly through flooding by sea water.
The 1970 Bhola cyclone
In November 1970, a powerful cyclone struck the delta of what was then East Pakistan and is now Bangladesh, near the Bay of Bengal’s head. A surge of several metres swept over low islands and coastal districts at night, and estimates of the death toll range from roughly 300,000 to 500,000 people. It is widely regarded as the deadliest tropical cyclone ever recorded. The disaster changed regional politics and led to major investments in warning systems and shelters in Bangladesh.
The 1977 Andhra Pradesh cyclone
In November 1977, a severe cyclone crossed the coast near the Krishna delta in Andhra Pradesh. A surge of around five metres inundated low-lying villages and islands such as those of Divi Seema, and the death toll was several thousand. The event pushed India to improve its cyclone warning and coastal protection.
The 1999 Odisha super cyclone
On 29 October 1999, a super cyclone made landfall near Paradip in Odisha after intensifying over a very warm Bay of Bengal. Winds were extreme, and the surge, commonly estimated at around six to nine metres, carried sea water many kilometres inland across the coastal plain. About ten thousand people died, and millions were affected. Most of the deaths were caused by drowning in the surge, and many victims had no cyclone shelter or reliable warning in reach.
More recent events
Cyclone Amphan in May 2020 struck the Sundarbans region of West Bengal and Bangladesh, breached embankments and flooded villages with salt water, though the loss of life was far lower than in past cyclones thanks to evacuations. Cyclone Biparjoy in 2023 and others on the west coast also tested Gujarat’s preparedness.
What decides how high a storm surge gets?
Storm surge height is decided by the cyclone’s intensity, size, speed and angle of approach, by the slope of the sea floor near the coast, the shape of the shoreline and the state of the tide at landfall. No single number such as wind speed fully predicts the surge.
| Factor | Effect on surge |
|---|---|
| Cyclone intensity | Stronger winds and deeper low pressure produce a higher surge |
| Storm size | A large storm pushes water over a bigger area and for longer, which can produce a higher surge than a small, compact one |
| Forward speed | A fast-moving storm can enhance the surge on its right side, while a slow one drives water for longer |
| Angle of approach | A storm hitting the coast head-on pushes more water than one running parallel to it |
| Sea floor and shelf | A wide, shallow shelf builds higher surges than a steep, deep one |
| Coastline shape | Bays, funnels and river mouths concentrate the water |
| Tide at landfall | High tide raises the total water level, and low tide softens the impact |
| Land elevation | Low delta land lets the water penetrate far inland |
This is why two cyclones of similar wind strength can produce very different flooding. A compact, fast cyclone hitting a steep coast may cause mostly wind damage, while a broad, slow one over a gentle delta shelf may cause a deadly surge.
How far inland can a storm surge travel?
A storm surge can travel several kilometres inland over flat coastal plains and much farther up rivers and creeks, and in the 1999 Odisha cyclone sea water reached well beyond the shoreline across low farmland. The distance depends on the surge height and on how gently the land rises.
On a very flat delta, each extra metre of surge can advance the flood by a great distance. Surge water also moves up river mouths and tidal creeks, pushing salt into rivers and flooding places that seem far from the sea. After the water retreats, salt often remains in the soil, damaging crops for seasons, and fresh water wells and ponds may turn brackish. Damage to embankments and mangroves can leave a coast vulnerable to the next storm.
The depth and speed of the flowing water are a danger in themselves. Fast, debris-filled water can sweep away light houses and people, and even a surge of a metre or two can drown people who are caught in a low village at night.
How do forecasters predict storm surge in India?
Forecasters predict storm surge in India using computer models that take the cyclone’s forecast track, intensity and size and calculate the resulting sea-level rise along the coast, and the India Meteorological Department includes the expected surge height in its cyclone warnings. The Indian National Centre for Ocean Information Services in Hyderabad provides ocean state and surge guidance as well.
The IMD is the designated regional specialised meteorological centre for tropical cyclones over the North Indian Ocean, and it issues track and intensity forecasts several days ahead, together with the expected height of the surge above the astronomical tide and the areas likely to be inundated. Hydrodynamic models, which use the coast’s depth and shape, convert the storm forecast into flood extent maps for districts, so that evacuation orders can be targeted. Tide gauges, buoys, radars and satellites help to check and update the forecasts.
Forecasting is not perfect. Small errors in track or landfall time can shift where and when the highest water arrives, and rapid intensification, when a storm strengthens quickly over warm water, can shorten warning time. For that reason, authorities often evacuate more widely than the central forecast requires.
How have shelters and evacuation reduced deaths?
Shelters and evacuation have reduced deaths dramatically because moving people out of the surge zone before landfall removes their exposure, and Odisha’s record shows the effect clearly, with about ten thousand dead in 1999 against very few in later comparable storms. Cyclone Phailin in 2013 and Fani in 2019 each saw roughly a million people or more moved to safety.
- Multipurpose cyclone shelters: raised, sturdy buildings that serve as schools or community halls in normal times and as refuges during storms.
- Early warning dissemination: alerts through mobile messages, radio, television, sirens and local volunteers in the local language.
- Mandatory evacuation: pre-planned movement of residents from low-lying villages using designated routes.
- Disaster management bodies: the Odisha State Disaster Management Authority and the National Disaster Management Authority coordinate with district teams and response forces.
- Community drills: practice and trained volunteers ensure people know where to go.
The successful pattern depends on trust, drills and good last-mile communication. It also requires attention to vulnerable groups such as the elderly, the disabled, pregnant women and fishers, and to livestock and livelihoods, which are often left out of evacuation plans.
What natural and engineered defences help?
Natural defences such as mangroves, coastal shelter-belts and dunes reduce wave energy and sometimes the reach of the surge, while embankments, raised roads and drainage systems protect specific areas. None of them stops a major surge, and each needs maintenance and sensible planning.
Mangrove forests like the Sundarbans and Bhitarkanika absorb wave energy and slow the water, which lowers damage behind them, although a very large surge can still overtop them. Embankments can protect farmland, but when they fail, as many did during Amphan, villages are flooded with salt water that cannot easily drain. Planners now combine hard and soft defences, safe building codes for coastal houses, raised plinths and early warning, in an approach known as integrated coastal management. Restricting construction in the most exposed strip along the shore, as set out in coastal regulation rules, also reduces the number of people at risk.
Will climate change make storm surges worse?
Climate change is expected to make storm surges more dangerous mainly because sea level is rising, so any given surge starts from a higher base, and because warmer seas may produce a larger share of intense cyclones. Scientists are less certain that the total number of cyclones will change.
Global sea level has risen by around twenty centimetres since the early twentieth century and is now rising by a few millimetres every year, with further rise projected. On deltas that are also sinking, the relative rise is faster, as the entry on land subsidence explains. Warmer ocean water supports quicker intensification of cyclones, and studies of the North Indian Ocean suggest that a larger proportion of storms are reaching severe categories, though the detailed trends are still being researched. Even without more storms, a metre of extra sea level would allow a given surge to flood much farther inland, so adaptation has to account for the future baseline, not only the past one.
Bottom Line
A storm surge is the cyclone’s water, not its wind, and it has been responsible for the worst losses of life on the Bay of Bengal coast, from the Bhola cyclone in 1970 to the Odisha super cyclone of 1999. India has shown that accurate forecasts, strong shelters, early evacuation and protected coasts can save lives, but rising seas and crowded coasts mean the preparation must keep improving. Last updated: 2 November 2026.
Frequently Asked Questions
What is a storm surge in simple words?
A storm surge is the extra sea water that a cyclone pushes onto the coast. It makes the sea level rise above the normal tide, and it can flood land several kilometres inland within hours. It is usually the most dangerous part of a cyclone.
Why is storm surge more dangerous than cyclone wind?
Wind damages buildings and trees, but the surge floods whole low-lying areas quickly, with deep, fast-moving water that can drown people. Historically, most cyclone deaths on the Bay of Bengal coast have been caused by flooding from the surge rather than by wind.
Which Indian coasts are most at risk of storm surge?
The most exposed coasts are West Bengal, Odisha and Andhra Pradesh on the east, because the Bay of Bengal’s shallow shelf and funnel shape raise surges. Gujarat on the west coast also faces risk, especially along creeks and low-lying areas.
How high was the surge in the 1999 Odisha super cyclone?
Estimates for the 1999 Odisha super cyclone put the surge at roughly six to nine metres near the landfall area close to Paradip. Sea water travelled many kilometres inland. The cyclone killed about ten thousand people, most of them through drowning.
How is a storm surge forecast in India?
The India Meteorological Department forecasts the cyclone’s track and strength and includes the expected surge height in its warnings, while the Indian National Centre for Ocean Information Services supports with ocean models. Computer models estimate the flooding for specific districts so that evacuation can be targeted.
What should people do when a storm surge warning is issued?
People in low-lying coastal areas should leave for the nearest cyclone shelter or higher ground as soon as the authorities order evacuation, and should not wait for the wind to rise. They should carry essentials and documents, follow official routes, and stay until the all-clear is given.
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